docs: def unique_reflections(self, hkl):
          """Returns a subset *hkl* containing only the symmetry-unique
          reflections.
  
          Example:
  
          >>> from ase.lattice.spacegroup import Spacegroup
          >>> sg = Spacegroup(225)  # fcc
          >>> sg.unique_reflections([[ 2,  0,  0], 
          ...                        [ 0, -2,  0], 
          ...                        [ 2,  2,  0], 
          ...                        [ 0, -2, -2]])
          array([[2, 0, 0],
                 [2, 2, 0]])
          """
          hkl = np.array(hkl, dtype=int, ndmin=2)
          hklnorm = self.symmetry_normalised_reflections(hkl)
          perm = np.lexsort(hklnorm.T)
          iperm = perm.argsort()
          xmask = np.abs(np.diff(hklnorm[perm], axis=0)).any(axis=1)
          mask = np.concatenate(([True], xmask))
          imask = mask[iperm]
          return hkl[imask]